An isolation circuit is used for electrically isolating an input side and an output side from each other. A transformer has primarily been known as an isolation circuit. Patent Document 1 discloses a technique pertinent to an isolation circuit using a piezoelectric transformer. FIG. 9 shows an example of a circuit that is an equivalent of circuitry using a transformer as an isolation circuit. The equivalent circuit has an input circuit 101, a transformer 102, and an output circuit 103. The input circuit 101 is circuitry for receiving a signal, and the output circuit 103 is circuitry for outputting a signal. The transformer 102 is also circuitry that effects electrical isolation.
The input circuit 101 has an AC power source 104 and input impedance 105, and the AC power source 104 is connected to a signal ground SG. A signal from the input circuit 101 is input to a first input port 106 of the transformer 102. A signal is output from a first output port 107 by action of electromagnetic coupling of the transformer 102. The signal is input to the output circuit 103. The output circuit 103 has a terminating resistor 108 and an ammeter 109. Accordingly, a signal is output from the output circuit 103.
In FIG. 9, in order to realize isolation in terms of the direct current, a signal ground SG is connected to a second input port 110 of the transformer 102, and a frame ground FG is connected to a second output port 111 of the transformer 102. Further, a capacitor 112 shown in FIG. 9 shows stray capacitance.